Roll libxml to 7279d236

This patch also adds --allow-empty to the git commit command for windows
in the roll.py script since there are usually no changes for windows.
libxslt's roll.py already does this for windows.

Change-Id: Ia75ef906c79abc907c396db5cc21f9e6ccb6f989
Bug: 934413
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2866731
Reviewed-by: Stephen Chenney <schenney@chromium.org>
Commit-Queue: Joey Arhar <jarhar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#879926}
NOKEYCHECK=True
GitOrigin-RevId: b0a66f930c41a1c5e8307c0f7996adc02ccd3cbd
29 files changed